RFID systems can be used to track and identify items. An RFID tag is a small electronic device that can be attached to an item, and can be coded with data identifying the item. An RFID interrogator reads the tags to identify the items. The RFID interrogator includes a transmitter to generate a transmitted signal used to search for the RFID tags, a receiver to receive backscattered signals from the RFID tags, and a decoder to decode the back scattered signals. The RFID tags receive an encoded transmitted signal from the interrogator, alter the signal, and return the signal to the interrogator via the principal of backscatter. The interrogator then receives the returned and altered signal and decodes it, thus identifying the item to which the tag is attached or associated with.
Passive RFID tags have no internal power source and use the energy of the interrogator's transmitted signal to power the tag. The transmitted signal is orders of magnitude stronger than the received signal being backscattered by the RFID tag. For example, the signal transmitted by the interrogator may have 1 Watt of power, while the received back scattered signal from the tag may have only 1 milliwatt (mW) of power or less. Since the interrogator is constantly powering the RFID tags in its vicinity, the interrogator's transmitter is always transmitting while the interrogator's receiver is simultaneously receiving the back scattered information from the tag. As a result of the interrogator's transmitter continuous operation, there are sources of transmitter energy that may leak into the receiver portion of the interrogator and distort the desired backscatter from the tag in the interrogator's receiver, which can cause the RFID tag to be misread or misidentified by the interrogator.
For example, an RFID interrogator may include a single antenna to simultaneously receive from and transmit signals to tags. The antenna is connected to a combiner. The combiner is a unidirectional device that routes signals from the interrogator's transmitter to the antenna only in one direction, and routes backscatter signals from tags through the antenna to the interrogator receiver in one direction only. Ideally, in the transmit direction, the combiner only transmits RF energy to the antenna, and RF energy coming from tag backscatter into the antenna will only be directed to the interrogator receiver. The combiner is not a perfect device, however, and transmitted RF energy will leak into the receiver, introducing distortion onto the received signal. This distortion can cause misidentification of the RFID tags. Therefore, what is needed is a way to remove the distortion introduced by the transmitted signal.